High-bit-rate digital subscriber line (HDSL) techniques are being used to provide higher bandwidth communication over existing wired connections, e.g., between a central office (CO) of a public switched telephone network (PSTN) and customer premises equipment (CPE) such as a computer, set-top box, telephone or private branch exchange (PBX). One such technique, known as HDSL2, is intended as a T1 replacement technology for the transport of 1.544 Mbps DS1 signals, and is used to connect two modems, e.g., a central office modem and a customer premises modem, over a single twisted pair connection. The HDSL2 line rate is 1.552 Mbps, which includes the 1.544 Mbps DS1 payload, bit stuffing, and HDSL2 frame overhead. The HDSL2 technique is described in greater detail in, e.g., T1E1.4/99-006R1-HDSL2 Draft Standard, March 1999, which is incorporated by reference herein.
It has been proposed that HDSL2 be configured to provide transport of synchronization information such as so-called “stratum 1” traceable synchronization, as described in ANSI T1.101-1994, Synchronization Interface Standard, 1994, which is incorporated by reference herein. Stratum 1 synchronization refers to the highest performance level clock in the four-level synchronization network described in the above-cited T1X1 document, and has a minimum long term accuracy of ±1×10−11.
The maximum variation in HDSL2 frame rate may be on the order of, e.g., ±32 parts-per-million (ppm), and is independent of the DS1 payload. An HDSL2 frame is typically synchronized by free-running clocks at each end of the HDSL2 connection. The above-noted HDSL2 bit stuffing is used to accommodate phase differences resulting from frequency differences between the HDSL2 frame and the DS1 payload. Unfortunately, this bit stuffing can add a significant amount of phase noise to the DS1 payload. In some cases, over 300 nanoseconds (ns) of phase variation may be introduced. However, in order to provide suitable timing in many important applications, the phase variation, in terms of maximum time interval error (MTIE), generally must be kept to about 60 ns or less under non-failure conditions.
Another problem associated with transport of synchronization over HDSL2 connections is the phase noise which is introduced by the DS1 payload itself. For example, the DS1 payload will in many cases be supplied to an HDSL2 mapper from a synchronous optical network (SONET) source. A significant amount of phase variation, e.g., up to 4.63 microseconds (μs) of phase variation, may be introduced as a result of the pointer mechanisms that SONET uses to control frequency differences between a SONET frame and the DS1 payload.
A known technique for eliminating the phase variation caused by HDSL2 mapping or SONET pointer adjustments involves the use of buffering. However, a significant drawback of a buffering approach is that such an approach often requires the addition of substantial delay that can interfere with the transport service of the payload. For example, in order to provide effective buffering sufficient to accommodate the above-noted maximum phase variation associated with SONET pointer mechanisms, a delay of at least 4.63 μs generally must be introduced, and such a delay will adversely affect the DS1 transport service.